Disk drive refreshing zones in segments to sustain target throughput of host commands

ABSTRACT

A disk drive is disclosed including a disk comprising a plurality of refresh zones, and a head actuated over the disk. The disk drive further comprises control circuitry for receiving access commands from a host. The control circuitry refreshes a refresh zone in a plurality of segments with an interval between each segment, and processes at least one of the access commands during the interval between at least two of the segments, wherein a size of each segment and the interval ensures an average throughput of access commands received from the host does not fall below a first threshold.

BACKGROUND

1. Field

The present invention relates to disk drives for computer systems. Inparticular, the present invention relates to a disk drive refreshingzones in segments to sustain target throughput of host commands.

2. Description of the Related Art

Disk drives comprise a disk and a head connected to a distal end of anactuator arm which is rotated about a pivot by a voice coil motor (VCM)to position the head radially over the disk. The disk comprises aplurality of radially spaced, concentric tracks for recording user datasectors and embedded servo sectors. The embedded servo sectors comprisehead positioning information (e.g., a track address) which is read bythe head and processed by a servo control system to control the velocityof the actuator arm as it seeks from track to track.

Because the disk is rotated at a constant angular velocity, the datarate is typically increased toward the outer diameter tracks (where thesurface of the disk is spinning faster) in order to achieve a moreconstant linear bit density across the radius of the disk. To simplifydesign considerations, the data tracks are typically banded togetherinto a number of physical zones, wherein the data rate is constantacross a zone, and increased from the inner diameter zones to the outerdiameter zones. This is illustrated in FIG. 1, which shows a prior artdisk format 2 comprising a number of data tracks 4, wherein the datatracks are banded together in this example to form three physical zonesfrom the inner diameter of the disk (ZONE 1) to the outer diameter ofthe disk (ZONE 3). The prior art disk format of FIG. 1 also comprises anumber of embedded servo sectors 6 ₀-6 _(N) recorded around thecircumference of each data track. Each servo sector 6 _(i) comprises apreamble 8 for storing a periodic pattern, which allows proper gainadjustment and timing synchronization of the read signal, and a syncmark 10 for storing a special pattern used to symbol synchronize to aservo data field 12. The servo data field 12 stores coarse headpositioning information, such as a track address, used to position thehead over a target data track during a seek operation. Each servo sector6 _(i) further comprises groups of servo bursts 14 (e.g., A, B, C and Dbursts), which comprise a number of consecutive transitions recorded atprecise intervals and offsets with respect to a data track centerline.The groups of servo bursts 14 provide fine head position informationused for centerline tracking while accessing a data track duringwrite/read operations.

During a write operation, a current is applied to a write element of thehead (e.g., a write coil) to create a magnetic field which magnetizesthe surface of the disk by orienting the direction of magnetic grains(e.g., horizontally in longitudinal magnetic recording, or vertically inperpendicular magnetic recording). The orientation of the grainsexhibits hysteresis thereby generating their own magnetic field when thewrite magnetic field is removed. During a read operation, a read elementof the head (e.g., a magnetoresistive element) transduces the magneticfield emanating from the disk surface into a read signal that isdemodulated into an estimated data sequence.

The hysteresis of the magnetic grains is not permanent meaning that overtime the grains will orientate into random directions (magnetic entropy)until the magnetic field is no longer sensed reliably (leading to dataerrors during reproduction). Magnetic entropy may also be precipitatedby various factors, such as increasing ambient temperature. That is, athigher temperatures the uniform alignment of the grains will degradefaster. Another factor that precipitates magnetic entropy is aphenomenon referred to as adjacent track interference (ATI) wherein whenwriting data to a target track, the fringe field from the write elementdegrades the uniform alignment of the grains recorded in an adjacenttrack. The degrading effect of ATI on the adjacent tracks compounds overtime with each write operation to the target track. Eventually, themagnetic field emanating from the disk surface will deteriorate to thepoint that the data is no longer recoverable.

To protect against catastrophic data loss due to magnetic entropy, theprior art has suggested to maintain refresh monitors for periodicallyrefreshing data within refresh zones (reading and rewriting data) so asto periodically realign the magnetic orientation of the grains. When arefresh monitor triggers, indicating that a refresh zone needsrefreshing, the prior art refreshes the entire zone which cansignificantly reduce or even halt the average throughput of accesscommands received from the host until the refresh operation is completedas illustrated in FIG. 2.

There is, therefore, a need to perform a refresh operation for a diskdrive while maintaining adequate throughput of host commands.

SUMMARY OF THE EMBODIMENTS OF THE INVENTION

An embodiment of the present invention comprises a disk drive includinga disk comprising a plurality of refresh zones, and a head actuated overthe disk. The disk drive further comprises control circuitry forreceiving access commands from a host. The control circuitry refreshes arefresh zone in a plurality of segments with an interval between eachsegment, and processes at least one of the access commands during theinterval between at least two of the segments, wherein a size of eachsegment and the interval ensures an average throughput of accesscommands received from the host does not fall below a first threshold.

In one embodiment, at least one of the refresh zones comprises aplurality of tracks.

In another embodiment, the tracks form a plurality of physical zones,and the segment size is selected relative to the physical zones. In oneembodiment, each track comprises a plurality of data sectors, and thesegment size is selected relative to a number of data sectors per trackwithin each physical zone. In another embodiment, each track comprises aplurality of data sectors, a number of skew data sectors is associatedwith a contiguous access pattern when switching tracks within one of thephysical zones, and the segment size is selected relative to the numberof skew data sectors.

In yet another embodiment, the control circuitry maintains a firstpriority queue of refresh zones scheduled to be refreshed using a firstinterval between segments, and maintains a second priority queue ofrefresh zones scheduled to be refreshed using a second interval betweensegments, wherein the second interval is shorter than the firstinterval. In one embodiment, a duration of the first interval ensures anaverage throughput of access commands received from the host does notfall below the first threshold, and a duration of the second intervalensures an average throughput of access commands received from the hostdoes not fall below a second threshold less than the first threshold. Inanother embodiment, the control circuitry maintains a refresh monitorfor each refresh zone, and transfers one of the refresh zones held inthe first priority queue to the second priority queue if thecorresponding refresh monitor exceeds a threshold.

In still another embodiment, the control circuitry adjusts the interval,and in one embodiment, the control circuitry maintains a command queueof access commands received from the host and adjusts the interval basedon a number of access commands in the command queue. In anotherembodiment, the control circuitry maintains a refresh queue of refreshzones scheduled to be refreshed and adjusts the interval based on anumber of refresh zones in the refresh queue.

Another embodiment of the present invention comprises a method ofoperating a disk drive, the disk drive as comprising a disk including aplurality of refresh zones, and a head actuated over the disk. Accesscommands are received from a host, and a refresh zone is refreshed in aplurality of segments with an interval between each segment, wherein atleast one access command is processed during the interval between atleast two of the segments, and a size of each segment and the intervalensures an average throughput of access commands received from the hostdoes not fall below a first threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format having a plurality of tracks bandedtogether to form a plurality of physical zones.

FIG. 2 illustrates how in the prior art the average throughput of accesscommands received from a host will fall to zero due to refreshing arefresh zone in one segment.

FIG. 3A shows a disk drive according to an embodiment of the presentinvention comprising a disk, a head actuated over the disk, and controlcircuitry.

FIG. 3B is a flow diagram executed by the control circuitry according toan embodiment of the present invention for refreshing a refresh zone insegments to ensure an average throughput of access commands receivedfrom the host does not fall below a first threshold.

FIGS. 4A and 4B illustrate how refreshing a refresh zone in segmentswith an interval between each segment ensures an average throughput ofaccess commands received from the host does not fall below a firstthreshold.

FIG. 5 illustrates an embodiment of the present invention wherein thesegment size is selected relative to a number of data sectors per trackand a number of skew data sectors within each physical zone.

FIG. 6 shows a refresh monitor according to an embodiment of the presentinvention wherein a refresh zone is inserted into a first priority queueand transferred to a higher priority queue if the refresh monitorindicates an exigent need to refresh.

FIGS. 7A and 7B show an embodiment of the present invention wherein asmaller interval is used to refresh zones in the higher priority queueto expedite the refresh process.

FIG. 8A is a flow diagram according to an embodiment of the presentinvention wherein the interval between segments of a refresh zone isadjusted relative to a number of access commands in a command queue.

FIG. 8B is a flow diagram according to an embodiment of the presentinvention wherein the interval between segments of a refresh zone isadjusted relative to a number of refresh zones in a refresh queue.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 3A shows a disk drive according to an embodiment of the presentinvention including a disk 2 having a plurality of refresh zones (e.g.,refresh zones 1-3), and a head 4 actuated over the disk 2. The diskdrive further comprises control circuitry 6 for receiving accesscommands from a host. The control circuitry 6 executes the flow diagramof FIG. 3B according to an embodiment of the present invention. When arefresh zone needs refreshing (step 8), the control circuitry 6refreshes the refresh zone in a plurality of segments (step 10) with aninterval between each segment, and processes at least one of the accesscommands during the interval between at least two of the segments (step12), wherein a size of each segment and the interval ensures an averagethroughput of access commands received from the host does not fall belowa first threshold. The refresh operation continues until all of thesegments of the refresh zone have been refreshed (step 14).

In the embodiment of FIG. 3A, the disk 2 comprises a plurality ofembedded servo sectors 16 ₀-16 _(N) that define a plurality of datatracks 18. The head 4 is connected to a distal end of an actuator arm 20which is rotated about a pivot by a voice coil motor (VCM) 22 in orderto actuate the head 4 radially over the disk 2. The control circuitry 6processes the read signal emanating from the head 4 to demodulate theinformation in the embedded servo sectors. The demodulated servoinformation (position error signal) is then processed (e.g., usingappropriate compensation filtering) to generate a position controlsignal 24 applied to the VCM 22 in order to position the head 4 over atarget data track (seek to the target data track and track thecenterline of the target data track). In one embodiment, each data trackis partitioned into a number of data sectors, wherein write/readoperations are performed on one or more data sectors at a time.

In the embodiment of FIG. 3A the data tracks 18 are banded together intoa plurality of refresh zones from the inner to outer diameter of thedisk. However, the refresh zones may be defined in any suitable manner,including to define the refresh zone relative to logical block addressesor physical block addresses. In one embodiment, the refresh zones mayextend across multiple disk surfaces.

FIGS. 4A and 4B illustrate how refreshing a refresh zone in segmentswith an interval between each segment ensures an average throughput ofaccess commands received from the host does not fall below a firstthreshold TH1. The average throughput may be measured in any suitablemanner, such as the number of access commands received per second. InFIG. 4A the y-axis represents the average throughput of host commands,and the x-axis represents time. In FIG. 4B, the y-axis represents anamount of data (segment size) that is refreshed between an interval T,and the x-axis represents time, wherein any suitable threshold TH1 andany suitable corresponding segment size and interval T may be selectedin the embodiments of the present invention. For example, the thresholdTH1 may be selected for a family of disk drives, or it may be configuredin-the-field relative to the needs of a particular user or a particularoperating environment. In an embodiment described below, the segmentsize and/or interval are adjustable relative to an operating parameterof the disk drive, such as the number of access commands in a commandqueue or a number of refresh zones queued for refreshing.

In one embodiment, the tracks form a plurality of physical zones similarto the prior art disk format shown in FIG. 1, wherein the physical zonesmay be the same or different than the refresh zones. A data rate of datarecorded in the data sectors may be increased from the inner diameterphysical zone to the outer diameter physical zone in order to maintain amore constant linear bit recording density. In one embodiment, thesegment size is selected relative the physical zones, for example, byselecting the segment size relative to a number of data sectors pertrack within each physical zone. In another embodiment, a number of skewdata sectors is associated with a contiguous access pattern whenswitching tracks within one of the physical zones, and the segment sizeis selected relative to a number of data sectors per track and thenumber of skew data sectors within each physical zone.

An example of the number of skew data sectors per physical zone isillustrated in FIG. 5 which shows that when switching tracks there maybe one skew data sector for physical zone one, two skew data sectors forphysical zone two, and three skew data sectors for physical zone three.In practice, there may be significantly more skew data sectors perphysical zone, and in one embodiment, they may be measured by a numberof wedges (e.g., the number of data sectors between consecutive servosectors 16). Since switching tracks consumes time, in one embodiment thesegment size is reduced by the switching latency. For example, in oneembodiment the segment size is selected as2×(data_sectors_per_track)−2.5×(skew_data_sectors). So if a particularphysical zone comprises 100 data sectors per track and 10 skew datasectors, the segment size would equal 2×(100)−2.5×(10)=175 data sectors.In one embodiment, once the segment size is selected, the interval T maybe selected for the segment size. The selected segment size and intervalT may result in a threshold TH1, i.e., a segment size and an intervalthat will ensure the average throughput of access commands received fromthe host does not fall below the first threshold TH1. In anotherembodiment, once the interval T is selected, the segment size may beselected relative to the interval T. The interval T and/or segment sizemay be fixed for a class of disk drives or may be adjusted based on diskdrive operating conditions, as will be described more fully below.

Any suitable refresh monitor may be employed in the embodiments of thepresent invention to determine when a refresh zone needs to berefreshed. FIG. 6 shows an example refresh monitor that may beimplemented by the control circuitry 6, comprising a first counter 26clocked at a frequency 28 that establishes a refresh interval, and asecond counter 30 clocked by a write signal 32 each time a writeoperation is performed to the refresh zone. Thus, the refresh monitor ofFIG. 6 can be considered as tracking a refresh interval that is biasedby the number of writes to the refresh zone to compensate for theprecipitating effect that write operations have on magnetic entropy. Theoutput of counter 26 is combined with the output of counter 30 at adder34, and when the output of adder 34 exceeds a first threshold 36 atcomparator 38, the corresponding refresh zone may be inserted into afirst queue having a first priority. Once the refresh zone is selectedfrom the first queue and refreshed, the counters 26 and 30 may be resetby a refresh signal 40. However, if the output of adder 34 exceeds asecond threshold 42 at comparator 44 before the refresh zone isrefreshed, then the refresh zone may be transferred from the first queueto a second queue having a higher priority in order to expedite therefresh operation for that refresh zone. That is, the refresh zonesstored in the second queue may be refreshed prior to the refresh zonesstored in the first queue.

In one embodiment, the control circuitry 6 maintains a first priorityqueue of refresh zones scheduled to be refreshed using a first intervalbetween segments, and maintains a second priority queue of refresh zonesscheduled to be refreshed using a second interval between segments,wherein the second interval is shorter than the first interval. In oneembodiment, a duration of the first interval ensures an averagethroughput of access commands received from the host does not fall belowthe first threshold, and a duration of the second interval ensures anaverage throughput of access commands received from the host does notfall below a second threshold less than the first threshold. Thisembodiment is illustrated in FIGS. 7A and 7B wherein a first refreshzone (ZONE 1) is selected from the first priority queue and refreshed ata first refresh interval T₁ that prevents the average throughput ofaccess commands received from the host from falling below a firstthreshold TH1. While refreshing the first refresh zone (ZONE 1) a secondrefresh zone (ZONE 2) is transferred from the first priority queue tothe second priority queue. The second refresh zone is then selected fromthe second priority queue and refreshed at a second interval T₂ thatprevents the average throughput of access commands received from thehost from falling below a second threshold TH2. Decreasing the intervalfrom T₁ to T₂ helps ensure that the refresh zones approaching theirrefresh limit are refreshed in a timely manner at the expense ofreducing the average throughput of access commands received from thehost.

In one embodiment, the control circuitry 6 may adjust the refreshinterval and/or the segment size using any suitable criteria. FIG. 8Ashows a flow diagram that may be executed by the control circuitry 6wherein the access commands received from the host are stored in acommand queue (step 46), and then the interval (and/or segment size) isadjusted based on a number of access commands in the command queue (step48). For example, the interval may be increased as more access commandsare queued in the command queue to help sustain the desired throughputof access commands. As the number of access commands in the commandqueue decreases, the interval may be decreased to help expedite therefresh of the queued refresh zones. Adjusting the interval and/orsegment size may also alter the threshold TH1 (FIG. 4A), such that, forexample, a shorter interval may result in the threshold TH1 decreasingdue to less access commands being processed.

FIG. 8B shows a flow diagram that may be executed by the controlcircuitry 6 according to another embodiment wherein a refresh zoneneeding to be refreshed (step 50) is inserted into a refresh queue (step52), and the control circuitry 6 adjusts the interval between segmentsbased on the number of refresh zones queued in the refresh queue (step54). For example, the interval may be decreased as the number of refreshzones queued for refreshing increases to ensure that the refresh zonesare processed in a timely manner. As described above, modifying theinterval may decrease or increase the threshold TH1 (FIG. 4A).

In another embodiment of the present invention, the refresh operationfor a refresh zone may be canceled or deferred based on any suitablecriteria, such as the number of access commands in the command queue. Inone embodiment, the refresh operation is restarted from the beginning ofthe refresh zone after being canceled, and in another embodiment, therefresh operation is restarted at the segment where the refreshoperation stopped (i.e., the refresh operation is deferred). In yetanother embodiment, the current segment being refreshed is finishedbefore deferring the refresh operation, the interval following thesegment is extended until enough access commands have been processed,and then the refresh operation continues. Accordingly, in one embodimentthe interval may be adjusted for each segment within a single refreshzone based on any suitable criteria, such as the number of accesscommands in the command queue. For example, if the interval weredecreased for each segment in the refresh zone, this may result in ahost average throughput curve that has a downward slope instead of thehorizontal line shown in FIG. 4A. This may result in a threshold TH1that is at the bottom of the sloped host average throughput curve.

Any suitable control circuitry 6 may be employed in the embodiments ofthe present invention, such as any suitable integrated circuit orcircuits. For example, the control circuitry 6 may be implemented withina read channel integrated circuit, or in a component separate from theread channel, such as a disk controller, or certain steps describedabove may be performed by a read channel and others by a diskcontroller. In one embodiment, the read channel and disk controller areimplemented as separate integrated circuits, and in an alternativeembodiment they are fabricated into a single integrated circuit orsystem on a chip (SOC). In addition, the control circuitry may include asuitable preamp circuit implemented as a separate integrated circuit,integrated into the read channel or disk controller circuit, orintegrated into an SOC.

In one embodiment, the control circuitry 6 comprises a microprocessorexecuting instructions, the instructions being operable to cause themicroprocessor to perform the steps of the flow diagrams describedherein. The instructions may be stored in any computer-readable medium.In one embodiment, they may be stored on a non-volatile semiconductormemory external to the microprocessor, or integrated with themicroprocessor in a SOC. In another embodiment, the instructions arestored on the disk 2 and read into a volatile semiconductor memory whenthe disk drive is powered on. In yet another embodiment, the controlcircuitry 6 comprises suitable logic circuitry, such as state machinecircuitry.

1. A disk drive comprising: a disk comprising a plurality of refreshzones including a first refresh zone and a second refresh zone, wherein:the first refresh zone comprises a plurality of tracks including a firsttrack; the second refresh zone comprises a plurality of tracks includinga second track; and a data rate of the second track is greater than adata rate of the first track; a head actuated over the disk; and controlcircuitry operable to: receive access commands from a host; refresh thefirst refresh zone in a first plurality of segments having a firstsegment size, wherein the first plurality of segments are refreshed witha first interval between each segment; refresh the second refresh zonein a second plurality of segments having a second segment size, whereinthe second plurality of segments are refreshed with a second intervalbetween each segment; and process at least one of the access commandsduring the first and second intervals; wherein the first segment size isdifferent from the second segment size.
 2. The disk drive as recited inclaim 1, wherein: the first track comprises a first number of datasectors; and the first segment size is based on the first number of datasectors.
 3. The disk drive as recited in claim 1, wherein: the firsttrack comprises a plurality first number of data sectors; a first numberof skew data sectors is associated with a contiguous access pattern whenswitching tracks within the first refresh zone; and the first segmentsize is based on the first number of skew data sectors.
 4. The diskdrive as recited in claim 1, wherein: the first track comprises a firstnumber of data sectors; a first number of skew data sectors isassociated with a contiguous access pattern when switching tracks withinthe first refresh zone; and the first segment size is based on the firstnumber of data sectors per track and the first number of skew datasectors.
 5. The disk drive as recited in claim 1, wherein the controlcircuitry is further operable to: maintain a first priority queue ofrefresh zones scheduled to be refreshed using a first interval betweeneach segment; and maintain a second priority queue of refresh zonesscheduled to be refreshed using a second interval between each segment,wherein the second interval is shorter than the first interval.
 6. Thedisk drive as recited in claim 5, wherein: a duration of the firstinterval ensures an average throughput of access commands received fromthe host does not fall below a first threshold; and a duration of thesecond interval ensures an average throughput of access commandsreceived from the host does not fall below a second threshold less thanthe first threshold.
 7. The disk drive as recited in claim 5, whereinthe control circuitry is further operable to: maintain a refresh monitorfor each refresh zone; and transfer one of the refresh zones held in thefirst priority queue to the second priority queue if the correspondingrefresh monitor exceeds a second threshold.
 8. The disk drive as recitedin claim 1, wherein the control circuitry is further operable to adjustat least one of the first and second intervals.
 9. The disk drive asrecited in claim 8, wherein the control circuitry is further operableto: maintain a command queue of access commands received from the host;and adjust at least one of the first and second intervals based on anumber of access commands in the command queue.
 10. The disk drive asrecited in claim 8, wherein the control circuitry is further operableto: maintain a refresh queue of refresh zones scheduled to be refreshed;and adjust at least one of the first and second intervals based on anumber of refresh zones in the refresh queue.
 11. A method of operatinga disk drive, the disk drive comprising a head actuated over a disk, thedisk comprising a plurality of refresh zones including a first refreshzone and a second refresh zone, wherein: the first refresh zonecomprises a plurality of tracks including a first track; the secondrefresh zone comprises a plurality of tracks including a second track;and a data rate of the second track is greater than a data rate of thefirst track; the method comprising: receiving access commands from ahost; refreshing the first refresh zone in a first plurality of segmentshaving a first segment size, wherein the first plurality of segments arerefreshed with a first interval between each segment; refreshing thesecond refresh zone in a second plurality of segments having a secondsegment size, wherein the second plurality of segments are refreshedwith a second interval between each segment; and processing at least oneof the access commands during the first and second intervals; whereinthe first segment size is different from the second segment size. 12.The method as recited in claim 11, wherein: the first track comprises afirst number of data sectors; and the first segment size is based on thefirst number of data sectors.
 13. The method as recited in claim 11,wherein: the first track comprises a first number of data sectors; afirst number of skew data sectors is associated with a contiguous accesspattern when switching tracks within the first refresh zone; and thefirst segment size is based on the first number of skew data sectors.14. The method as recited in claim 11, wherein: the first trackcomprises a first number of data sectors; a first number of skew datasectors is associated with a contiguous access pattern when switchingtracks within the first refresh zone; and the first segment size isbased on the first number of data sectors per track and the first numberof skew data sectors.
 15. The method as recited in claim 11, furthercomprising: maintaining a first priority queue of refresh zonesscheduled to be refreshed using a first interval between each segment;and maintaining a second priority queue of refresh zones scheduled to berefreshed using a second interval between each segment, wherein thesecond interval is shorter than the first interval.
 16. The method asrecited in claim 15, wherein: a duration of the first interval ensuresan average throughput of access commands received from the host does notfall below a first threshold; and a duration of the second intervalensures an average throughput of access commands received from the hostdoes not fall below a second threshold less than the first threshold.17. The method as recited in claim 15, further comprising: maintaining arefresh monitor for each refresh zone; and transferring one of therefresh zones held in the first priority queue to the second priorityqueue if the corresponding refresh monitor exceeds a threshold.
 18. Themethod as recited in claim 11, further comprising adjusting at least oneof the first and second intervals.
 19. The method as recited in claim18, further comprising maintaining a command queue of access commandsreceived from the host; and adjusting at least one of the first andsecond intervals based on a number of access commands in the commandqueue.
 20. The method as recited in claim 18, further comprising:maintaining a refresh queue of refresh zones scheduled to be refreshed;and adjusting at least one of the first and second intervals based on anumber of refresh zones in the refresh queue.
 21. A disk drive ascomprising: a disk comprising a plurality of refresh zones including afirst refresh zone and a second refresh zone, wherein: the first refreshzone comprises a plurality of tracks including a first track; the secondrefresh zone comprises a plurality of tracks including a second track;and a data rate of the second track is greater than a data rate of thefirst track; a head actuated over the disk; a means for receiving accesscommands from a host; a means for refreshing the first refresh zone in afirst plurality of segments having a first segment size, wherein thefirst plurality of segments are refreshed with a first interval betweeneach segment; a means for refreshing the second refresh zone in a secondplurality of segments having a second segment size, wherein the secondplurality of segments are refreshed with a second interval between eachsegment; a means for processing at least one of the access commandsduring the first and second intervals; and wherein the first segmentsize is different from the second segment size.